1. Field of the Invention
This invention is directed to an optical out-of band vehicle for transporting optical channel overhead information.
2. Background Art
Traditionally, the performance of synchronous optical networks is monitored using bytes with specific assignation, carried in an overhead associated with the payload. For example, the information in the SONET transport overhead (TOH) is used by the SONET equipment for control and signalling purposes. Thus, TOH comprises fields indicating the order of the tributaries into a higher rate signal, error check fields, frame alignment information, administrative error fields and signalling information.
Signalling in SONET may be accomplished using a 3-Byte data communication channel (DCC) field in the section overhead that provides a 192 Kb/s message channel used from a central location for operation, administration and maintenance (OAM) between section entities. Section entities are for example optical amplifiers and repeaters. A 9-Byte DCC field is also provided in the line overhead, that offers a 576 Kb/s message channel for OAM information between line entities. Line entities are for example add/drop multiplexers.
The DCC channels are available for internally generated, externally generated and manufacturer specific messages, such as alarms, controls, OAM and other communication needs,
As the synchronous networks evolved to carry payloads with various types of traffic (e.g ATM, IP), the amount of overhead information (OH) also evolved. The overhead information includes optical channel overhead information (OCH), optical multiplex overhead information and optical transport overhead.
As a result of these requirements, additional ways of conveying the OH information were proposed and some are in use. For example, a known way of transporting OH information is to use an in-band optical service channels (OSC), which can have a unidirectional or a bidirectional implementation. An OSC is used exclusively between two sites equipped with optical amplifiers, to carry optical section transport overhead information, and it has a rate of 4.86 Mb/s.
It is also known to use wavelength identifiers for each channel, to partially verify connectivity at optical multiplexers. This is done, for example, by on-off keying of a dither to modulate the optical signal at a low rate. The same dither also provides sometimes information about the power of the respective channel.
However, while the above solutions are suitable to manage the current opaque SONET networks, they cannot accommodate the amount of information needed for overhead of next generation of optical transparent networks.
As the service channels are carried in-band for SONET/SDH systems, the timing information is available at the ends of the channel, so that the data can be used without additional clock circuitry. However, a service channel may not be in band for other type of signals than SONET, which means that additional clock circuitry must be provided at the nodes where the information carried by the service channel is needed. This is an important deterrent for the emerging transparent networks.
Co-pending patent application Ser. No. 09/218053, filed on Dec. 2, 1998 by Northern Telecom Limited and entitled xe2x80x9cApparatus and method for versatile digital communicationxe2x80x9d (Habel et al.) discloses a simple phase locked loop (PLL), that enables clock recovery for any bit-rate and operates over a broad continuous range of frequencies. The PLL may be used to provide 3R transponder/regenerators with read/write access to overhead data, eye quality assessment, monitoring/identification of the bit-rate of the signal, and with bit-rate independent digital multiplexing and demultiplexing functions. Such a frequency agile 3R transponder/regenerator may also enable use of a high-speed out-of band vehicle for transporting the optical channel overhead. The above-identified patent application is incorporated herein by reference.
It is an object of the present invention to provide a high-speed out-of band vehicle for communicating overhead information between network entities of a transparent optical network, that alleviates totally or in part the drawbacks of the prior art solutions.
The main advantage of this out-of band vehicle is that it allows management of next generation optical transparent networks with at least the same rich features used today to manage opaque SONET networks.
The out-of-band vehicle for OH data can be readily used with the transparent receivers, transceivers or transponders described in the patent application Ser. No. 09/218053, providing for network elements that may be managed with minimal interference with the payload. In addition, as there are no restrictions on the format and rate of the OH data, a network provider is granted the option of selecting the type and amount of OAMandP operations it wishes.
Still another advantage of the out-of-band vehicle for OH data is that the interference with the payload is minimal, the clock recovery is simple and there are few additional components needed to implement this service channel.